In the packaged power industry, there is an ever increasing emphasis upon the development of high capacity, small volume electric cells. The following is a comparison of the capacity and voltage for some active materials currently used in commercially available cells:
______________________________________ Active EMF vs. Zn in Capacity Material Alkaline Electrolyte ma-hr./g amp-hr./cc ______________________________________ HgO 1.35v. 248 2.76 Ag.sub.2 O 1.60v. 232 1.67 AgO 1.82v. 432 3.22 ______________________________________
Divalent silver oxide (AgO) is an excellent high capacity battery active material, but it has two properties which have limited its use as a battery active material. During the discharge of a battery employing a divalent silver oxide positive active material, the initial voltage is at the higher divalent voltage level (1.82v. vs. Zn in alkaline electrolyte) until substantially all of the AgO is converted to Ag.sub.2 O, and thereafter, the discharge continues at the lower monovalent voltage level (1.60v. vs. Zn in alkaline electrolyte). This two plateau voltage level during discharge cannot be tolerated by many types of battery operated equipment.
Another problem encountered when using divalent silver oxide as a depolarizer (positive active material) is its lack of stability when in contact with aqueous alkaline solutions. It is well known that divalent silver oxide evolves oxygen when in contact with aqueous alkaline solutions, and this gassing phenomenon causes self-discharge of the divalent silver oxide, converting it to monovalent silver oxide or metallic silver. Divalent silver oxide cannot be used as the positive active material in hermetically sealed cells because of this instability in alkaline solutions and the consequent hazard of pressure build-up and possible cell rupture.
The problem of the two plateau voltage level during the electrical discharge of divalent silver oxide has previously been overcome by the inventions disclosed in U.S. Pat. Nos. 3,615,858 and 3,655,450 issued to Luis Soto-Krebs. These patents disclose a battery having a positive electrode comprising a principal active material (e.g., divalent silver oxide) and a secondary active material (e.g., monovalent silver oxide) whose discharge product is readily oxidized by the principal active material in the presence of alkaline electrolyte, and wherein the sole electronic path for discharge of the principal active material is through the secondary active material. The battery invented by Soto-Krebs is characterized throughout discharge by the potential of the secondary active material (Ag.sub.2 O) vs. the negative electrode in the alkaline electrolyte. The battery has the advantage of a single voltage level during electrical discharge and also the increased capacity provided by the divalent silver oxide positive active material.
The problem of the divalent silver oxide instability has been overcome by the inventions disclosed in U.S. Pat. Nos. 3,476,610 and 3,484,295 issued to Luis Soto-Krebs and Robert Dawson. These patents disclose a battery having a positive electrode comprising a principal active material (e.g., divalent silver oxide) and a secondary active material (e.g., monovalent silver oxide) employed as a substantially electrolyte-impermeable layer interposed between the principal active material and the battery components containing the electrolyte. This construction isolates the principal active material from contact with the electrolyte until the secondary active material is discharged, thereby providing improved stand or shelf life.